The piston of a reciprocated engine generally has a skirt formed underneath the piston head, this skirt being connected to a connecting rod via a piston pin passing through the skirt. This skirt has the function of supporting the piston so as to prevent it from inclining under the force of the combustion energy acting upon it, and it slides on the inner circumference of the cylinder.
As the skirt suffers considerable thermal expansion in a direction of the piston pin due to the combustion heat of the engine, it may for example be formed with an elliptical cross-section having the short axis on the direction of the piston pin, and the long axis on the direction perpendicular to the piston pin.
Although the skirt supports the piston, it should not increase the frictional resistance of the piston. For this reason, it is desirable that the area of contact between the skirt and the cylinder is kept small to the extent that the necessary support is not lost.
For this purpose, Tokkai Sho 58-38357 published by the Japanese Patent Office discloses a piston having a projection to promote contact between the skirt and the piston.
Further, in Japanese Patent Applications Tokugan Hei 1-250527 and Tokugan Hei 1-335753, this Applicant has proposed varying the shape on one side of the piston pin, and a piston having a T-shaped projection.
However, if the skirt is given a projection in this way, contact pressure with the cylinder tends to concentrate on the edge of the projection. As a result, the thin film of lubricating oil on the inner surface of the cylinder breaks easily, and there is a risk of increasing frictional resistance between the piston and cylinder or of increasing local wear. Further, if the pressure is unevenly distributed on the edge, the piston may seize up if it slides slowly when the contact pressure with the cylinder is large as when the engine is running under low speed, high load conditions.